Various adhesive compositions have been used to adhere substrates. Compositions used in the art include water-borne adhesives, solvent borne adhesives and hot melt adhesives. Hot melt adhesives are advantageous because they are non-polluting as compared to solvent based systems, and do not require evaporation of water as in water based systems. Hot melt adhesives, based on thermoplastics have proven useful in assembly line applications where speed is essential, but the known systems have proven to have disadvantages.
Commercial hot melt adhesives based on polyamides have limited low temperature capacity. For instance, polyamides such as Nylon 6 and Nylon 6-12 have glass transition temperatures of about 65.degree. C. and 45.degree. C., respectively. These polyamides become brittle at temperatures below 0.degree. C. They are therefore excluded from bonding applications, for instance in automotive and refrigeration applications, where flexibility of adhesives at temperatures as low as -40.degree. C. is required. Polyamides derived from a dimerized fatty acid monomer can extend the low temperature capacity further, however, they are usually limited to temperatures as low as -20.degree. C. In addition, these kinds of polyamides usually have low adhesion strength and fail undesirably under loads by the adhesive failure mode. (This means that failure occurs at the interface between the substrate and the adhesive.) An example of such a polyamide adhesive is disclosed in European Patent Publication EP 46968 dated Mar. 10, 1982. The disclosed polyamide is made by polymerizing dimerized fatty acids with diglycolamine and other compounds.
Polyetheramide block copolymers with useful application temperatures (-40.degree. C. to 150.degree. C.) usually have very high melt viscosities (.apprxeq.9,000 poise at 200.degree. C.) and are not applicable as hot melt adhesives due to the limitations of the capacity of hot melt dispensing systems. Commercial hot melt dispensing systems generally cannot handle materials having viscosities greater than 1000 to 2000 poises. Hot melt adhesives with dispersing temperatures higher than 200.degree. C. are not suitable to bind many thermoplastic substrates, such as polycarbonate, and the like. Thus, the foregoing polyetheramides are not known as adhesives.
Belgian Pat. No. 891,543 discloses a polyetheramide said to be a hot melt adhesive which can be used to bond aluminum to aluminum. However, the molecular weight of the polymer is reported to be 1000. Such a polyetheramide is distinctive from the polyetheramides contemplated with respect to the present invention which have molecular weights greater than 10,000, on the order of 15,000 to 30,000.
Other attempts at utilizing the polyetheramides in hot melt compositions appear in French Patent No. 2,533,577, which discloses a complex mixture of components including a polyetheramide, but also including butyl rubber, a terpene-phenolic resin, an ethylene vinyl acetate copolymer, a polyisobutylene and polypropylene. There is no disclosure or suggestion to use a polyamide in the mixture. Similarly, French Patent No. 2,523,143 discloses a mixture of a polyetheramide with a vinyl ester polymer, but does not suggest the use of a polyamide.
Other references which disclose a polyetheramide and make tangential references to a polyamide include Japanese Kokai 60/960 dated Jan. 7, 1985, which discloses tubes having separate layers of a polyetheramide and a polyamide, but which do not disclose any admixture of the two types of polymers. A related disclosure is found in Japanese Kokai 59/129137 dated July 25, 1984. A polyetheramide is used as a carrier for a dye in improving the dyeing characteristics of polyamide fibers, in Research Disclosure, Volume 229, pages 175 to 176, (published in the United Kingdom by Kenneth Mason Publications, Ltd.)
Therefore, one objective of the present invention is to develop hot melt adhesives which can be processed or dispensed using commercially available hot melt systems. Another objective of the present invention is to provide a hot melt adhesive which has a higher adhesive strength and a reliable adhesion failure mode (cohesive failure mode). This means that the failure occurs within the body of the adhesive, and is the preferred mode of failure, compared to the adhesive failure mode. Furthermore, another objective of the present invention is to make a hot melt adhesive which has good oil resistance for automotive under the hood applications.